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//===- ObjCARCAPElim.cpp - ObjC ARC Optimization --------------------------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
/// \file
/// This file defines ObjC ARC optimizations. ARC stands for Automatic
/// Reference Counting and is a system for managing reference counts for objects
/// in Objective C.
/// This specific file implements optimizations which remove extraneous
/// autorelease pools.
/// WARNING: This file knows about certain library functions. It recognizes them
/// by name, and hardwires knowledge of their semantics.
/// WARNING: This file knows about how certain Objective-C library functions are
/// used. Naive LLVM IR transformations which would otherwise be
/// behavior-preserving may break these assumptions.
#include "ObjCARC.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/PassManager.h"
#include "llvm/InitializePasses.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/ObjCARC.h"
using namespace llvm;
using namespace llvm::objcarc;
#define DEBUG_TYPE "objc-arc-ap-elim"
namespace {
/// Interprocedurally determine if calls made by the given call site can
/// possibly produce autoreleases.
bool MayAutorelease(const CallBase &CB, unsigned Depth = 0) {
if (const Function *Callee = CB.getCalledFunction()) {
if (!Callee->hasExactDefinition())
return true;
for (const BasicBlock &BB : *Callee) {
for (const Instruction &I : BB)
if (const CallBase *JCB = dyn_cast<CallBase>(&I))
// This recursion depth limit is arbitrary. It's just great
// enough to cover known interesting testcases.
if (Depth < 3 && !JCB->onlyReadsMemory() &&
MayAutorelease(*JCB, Depth + 1))
return true;
return false;
return true;
bool OptimizeBB(BasicBlock *BB) {
bool Changed = false;
Instruction *Push = nullptr;
for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) {
Instruction *Inst = &*I++;
switch (GetBasicARCInstKind(Inst)) {
case ARCInstKind::AutoreleasepoolPush:
Push = Inst;
case ARCInstKind::AutoreleasepoolPop:
// If this pop matches a push and nothing in between can autorelease,
// zap the pair.
if (Push && cast<CallInst>(Inst)->getArgOperand(0) == Push) {
Changed = true;
LLVM_DEBUG(dbgs() << "ObjCARCAPElim::OptimizeBB: Zapping push pop "
"autorelease pair:\n"
" Pop: "
<< *Inst << "\n"
<< " Push: " << *Push
<< "\n");
Push = nullptr;
case ARCInstKind::CallOrUser:
if (MayAutorelease(cast<CallBase>(*Inst)))
Push = nullptr;
return Changed;
bool runImpl(Module &M) {
if (!EnableARCOpts)
return false;
// If nothing in the Module uses ARC, don't do anything.
if (!ModuleHasARC(M))
return false;
// Find the llvm.global_ctors variable, as the first step in
// identifying the global constructors. In theory, unnecessary autorelease
// pools could occur anywhere, but in practice it's pretty rare. Global
// ctors are a place where autorelease pools get inserted automatically,
// so it's pretty common for them to be unnecessary, and it's pretty
// profitable to eliminate them.
GlobalVariable *GV = M.getGlobalVariable("llvm.global_ctors");
if (!GV)
return false;
assert(GV->hasDefinitiveInitializer() &&
"llvm.global_ctors is uncooperative!");
bool Changed = false;
// Dig the constructor functions out of GV's initializer.
ConstantArray *Init = cast<ConstantArray>(GV->getInitializer());
for (User::op_iterator OI = Init->op_begin(), OE = Init->op_end();
OI != OE; ++OI) {
Value *Op = *OI;
// llvm.global_ctors is an array of three-field structs where the second
// members are constructor functions.
Function *F = dyn_cast<Function>(cast<ConstantStruct>(Op)->getOperand(1));
// If the user used a constructor function with the wrong signature and
// it got bitcasted or whatever, look the other way.
if (!F)
// Only look at function definitions.
if (F->isDeclaration())
// Only look at functions with one basic block.
if (std::next(F->begin()) != F->end())
// Ok, a single-block constructor function definition. Try to optimize it.
Changed |= OptimizeBB(&F->front());
return Changed;
/// Autorelease pool elimination.
class ObjCARCAPElim : public ModulePass {
void getAnalysisUsage(AnalysisUsage &AU) const override;
bool runOnModule(Module &M) override;
static char ID;
ObjCARCAPElim() : ModulePass(ID) {
} // namespace
char ObjCARCAPElim::ID = 0;
INITIALIZE_PASS(ObjCARCAPElim, "objc-arc-apelim",
"ObjC ARC autorelease pool elimination", false, false)
Pass *llvm::createObjCARCAPElimPass() { return new ObjCARCAPElim(); }
void ObjCARCAPElim::getAnalysisUsage(AnalysisUsage &AU) const {
bool ObjCARCAPElim::runOnModule(Module &M) {
if (skipModule(M))
return false;
return runImpl(M);
PreservedAnalyses ObjCARCAPElimPass::run(Module &M, ModuleAnalysisManager &AM) {
if (!runImpl(M))
return PreservedAnalyses::all();
PreservedAnalyses PA;
return PA;